It's nice to able to search, browse and download mobile apps from your computer as well as your handset. (See: iTunes.) Google doesn't seem to realise or care about this, so third-party syncing app DoubleTwist picked up their slack.
In theory, this is Android's iTunes: It's a place where Android users can sift through apps on a larger screen, with a sightly more accommodating interface. In reality, it's a bit less: It doesn't offer many sorting or navigation options that Android App Market browser doesn't already have, and you can't actually download apps for syncing to your device. (The closest you can get to downloading an app in DoubleTwist is to use your phone to scan the QR code displayed on the app's page, which is pretty convenient, but not as convenient as queuing it directly for installation, like you can do with iTunes.)
It's a respectable effort from an outfit that doesn't even have Google blessing, but it really just drives the point home that Google should provide something like this themselves. I understand that Android handsets are supposed to exist in a computerless vacuum, and that email, contacts and other lightweight data sync perfectly well over the air, with the cloud. But it's hard to overstate how important apps are to a smartphone OS's success, and just as hard to overstate how much nicer it is to have a decent interface for the thousands of apps that make Android great. So, Google: Try it. Or at least help these guys out. DoubleTwist for Mac supports the Android Market now; support will be added to the Windows version soon. [DoubleTwist]